Zirconia sintered bodies, in which yttria is solid-solved as a stabilizer, are used as dental materials such as dentures, by laminating porcelain or the like on the surface of the zirconia sintered body so as to adjust the color tone to be equivalent to that of a natural tooth. On the other hand, there is an increasing demand for zirconia sintered bodies which can be used directly as dental materials without laminating porcelain or the like. When a zirconia sintered body is used as a dental material, not only are mechanical properties of strength and toughness considered to be necessary, but optical characteristics such as translucency and color tone equivalent to those of a natural tooth are also required from an aesthetic standpoint.
For example, Patent Document 1 discloses a zirconia sintered body comprising from 2 to 4 mol % of yttria, wherein the alumina content is not greater than 0.2 wt. %, and the total light transmittance at a thickness of 1 mm is not less than 35%. A sintered body having a total light transmittance of 41% (total light transmittance of 36% for light having a wavelength of 600 nm at a thickness of 1.0 mm) disclosed in the working example has translucency and strength suitable for a back tooth denture. However, the color tone of this sintered body is white, so the sintered body cannot be used alone as a front tooth denture.
In addition, Patent Document 2 discloses a translucent zirconia sintered body which is colored yellow by adding Fe. This sintered body has a color tone closer to that of a natural tooth than a zirconia sintered body that does not contain Fe. However, this sintered body still has a color tone that differs from that of a natural tooth. Because the color tone difference between the sintered body and a natural tooth is substantial, as described above, the sintered body cannot be used alone as a denture.
Patent Document 3 discloses a zirconia sintered body containing from 1.5 to 5 mol % of yttria and having a porosity of not greater than 0.6%. However, this sintered body is a zirconia sintered body obtained by pressure sintering using hot isostatic pressing (also called “HIP” hereafter). In addition, Patent Document 4 discloses a zirconia sintered body containing yttria in an amount of greater than 4 mol % and not greater than 7 mol % and having a total light transmittance of not less than 40% for light having a wavelength of 600 nm at a thickness of 1 mm. This sintered body is also a zirconia sintered body obtained by pressure sintering. Furthermore, Non-Patent Document 1 discloses a transparent zirconia sintered body obtained by performing spark plasma sintering (called “SPS” hereafter) on a zirconia powder containing 3 mol % of yttria or 8 mol % of yttria.
These special sintering methods such as pressure sintering or SPS increase the production cost of the zirconia sintered bodies. Therefore, these zirconia sintered bodies can only be used in limited applications. Moreover, the transparency of the zirconia sintered body disclosed in Patent Document 4 or Non-Patent Document 1 is too high. Therefore, these sintered bodies give an unnatural impression as a front tooth denture.
In addition, the method ordinarily used to produce a denture from a translucent zirconia sintered body is to mold a zirconia powder, to calcine the zirconia powder at a temperature equal to or lower than the sintering temperature of zirconia, to process the resultant pre-sintered body into the shape of a denture, and to sinter the processed pre-sintered body at the sintering temperature of zirconia. Therefore, a zirconia powder with which a high-density zirconia sintered body that can be obtained by sintering under normal pressure for a short amount of time is desirable.
Conventionally, in order to achieve the same aesthetics as a natural tooth, porcelain or the like is laminated on the surface of a zirconia sintered body so as to adjust the color tone, and this is used as a dental material (for example, see Patent Document 5). This dental material is a composite material comprising zirconia and a glass material differing in strength from zirconia. The strength of such a composite material was not sufficient for a dental material.
Therefore, zirconia sintered bodies for dental materials which make it possible to enhance aesthetics while maintaining strength, without laminating porcelain or the like, are being researched.
For example, zirconia sintered bodies having the same translucency as a natural tooth have been reported (Patent Document 6). The sintered bodies disclosed in Patent Document 6 are used directly as a dental material. These zirconia sintered bodies have the same translucency as a natural tooth. On the other hand, these sintered bodies have a different color tone than a natural tooth—that is, the original bright white color tone of zirconia.
In addition, a colored zirconia sintered body for a dental material containing oxides as a colorant has been reported (for example, Patent Document 7). However, this sintered body is obtained by adding a rare earth oxide acting as a stabilizer to zirconia as a raw material for a base composition by means of powder mixing. Therefore, such a colored zirconia sintered body has very low strength.